The present invention relates generally to the encoding and decoding of digital data using a transcoder, and more specifically to the insertion of a translucent logo into the encoded data stream.
Translucent logos are typically integrated into a video signals using analog technology. An analog rendering of the translucent logo is used as a foreground and superimposed over a separate analog video image background, and the two images are combined to provide the video image incorporating the translucent logo. If the video image had been digitized, the digital video image had to be converted to an analog signal so that the analog rendering of the translucent logo could be inserted in the foreground. After the insertion of the translucent logo, the analog combination of the video image and the translucent logo was typically digitized for further transmission. This conversion of the video image from digital to analog and back to digital is obviously inefficient, and can result in a loss of picture quality.
A video bitstream digitized, encoded and compressed in accordance with MPEG standards conserves space and memory by re-using video images. Instead of creating the entire video image for each video frame, the MPEG signal instead generates individual sections of the video image once and moves the sections as required from frame to frame using motion vectors (MVs). The video image is typically organized in the form of MacroBlocks (MBs), each of which can be an 8xc3x978 block of pixels. An MPEG transcoder may be used to modify the digital data to change the bit-rate of the encoded signal. An example of such an MPEG transcoder is described in Keesman et al., xe2x80x9cTranscoding of MPEG bitstreamsxe2x80x9d, Signal Processing: Image Communications vol 8, pp. 481-500 , 1996. Such a transcoder re-uses the motion vectors without change in the reencoded signal, and minimizes changes to the macroblock mode. No technique has heretofore existed for incorporating a translucent logo directly into the MPEG bitstream.
The present invention provides a method and apparatus by which a translucent logo is inserted into the transcoded digital bitstream of an MPEG transcoder without changing the digital nature of the bitstream. Such an MPEG transcoder has cascaded decoding and encoding sections for the digital bitstream. In the present invention, the translucent logo is generated and is added to the transcoded bitstream of the MPEG transcoder upstream of the transcoder""s encoding section. The encoding section of the transcoder then encodes the bitstream which includes the translucent logo.
In a first embodiment of the present invention, the motion vectors from the original encoded video image are used in the reencoded image which now includes the translucent logo. This method is consistent with normal MPEG transcoder operation in which the transcoder re-uses the motion vectors, and is thus quite efficient. However, re-using the motion vectors intended for sections of the video image now occupied in whole or in part by the translucent logo does not produce a completely stable logo. This embodiment has the advantage of minimal complexity balanced with a slight potential loss in video quality.
A second embodiment of the present invention employs a recalculation of the motion vectors used in the reencoded bitstream. This recalculation is based upon the area of each macroblock taken up by the translucent logo relative to that of the underlying picture. A threshhold value is designated in the second embodiment of the present invention to dictate what percentage of the area of a macroblock can be covered by the translucent logo without changing the motion vector for the macroblock. When the logo coverage in a macroblock is more than the threshhold value, the motion vector for that macroblock is set to zero. If not, then the motion vector for that macroblock remains unchanged from its original value. The second embodiment adds a certain level of complexity to the transcoder function, but may improve video quality over the first embodiment.
The first and second embodiments share the advantage of introducing a transparent logo to the video bitstream as it passes through the MPEG transcoder. The necessity in the prior art of reducing the bitstream to analog, and superimposing an analog translucent logo, is eliminated completely. This yields significant benefits for services which seek to superimpose information on an existing video bitstream, such as a cable television operation which wishes to add information to an existing satellite video feed.
The novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.